KVM: s390: vsie: Move conditional reschedule

Let's move it to the outer loop, in case we ever run again into long
loops, trying to map the prefix. While at it, convert it to cond_resched().

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200403153050.20569-5-david@redhat.com


Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>